The present invention relates to a laser scanning optical system to be employed in an image depicting device such as a laser plotter, and more particularly to a laser scanning optical system capable of restraining appearance of a stationary ghost image on a scanned surface.
In a laser scanning optical system of an image depicting device, generally, a light beam (a laser beam) emitted from a laser source is deflected by a deflector such as a polygonal mirror, and led to a target surface (i.e., a surface to be scanned) via an imaging optical system such as an f.theta. lens for scanning the target surface.
In this specification, a light beam which is directly incident from a laser source on one reflection surface of the polygonal mirror and reflected thereby to reach a target surface is called as a "normal" light beam.
A part of the normal light beam is diffuse-reflected on a target surface and returned to the polygonal mirror through the imaging optical system. If thus returned light is reflected by other reflection surface(s) of the polygonal mirror and led to the target surface again through the imaging optical system, a ghost image is formed on the target surface at a position different from that of an image formed by the normal light beam. In this specification, light which forms a ghost image is called as a "ghost" light.
A position where the ghost image is formed is fixed on the target surface despite positional change of a spot of the normal light beam on the target surface for scanning, since influence due to movement of a spot on a target surface is cancelled out with influence due to rotation of a polygonal mirror. In this specification, such a ghost image, whose position on the target surface is fixed, is called as a "stationary ghost."
Japanese Patent Provisional Publication No. SHO 58-68014 discloses an arrangement of optical elements for canceling a stationary ghost formed by a reflection surface next to the reflection surface on which the normal light beam is incident. In the JP publication, an angle .alpha. formed between a principal ray of a light beam incident on a polygonal mirror and an optical axis of an imaging optical system (i.e., an f.theta. lens) on a main scanning plane satisfies the following condition: EQU .alpha.&lt;(4/N)-(W/D)
where, N represents the number of reflection surfaces of a polygonal mirror;
W represents a distance, on a target surface, from the optical axis of the imaging optical system to the end of an effective scanning range; and PA1 D represents a distance between a target surface and a target surface side principal point of the imaging optical system. PA1 N represents the number of reflection surfaces of the polygonal mirror; PA1 f represents a focal length (unit:mm) of the imaging optical system in a principal scanning direction; PA1 W1 represents a position (unit: mm) on said scanning target surface with respect to the optical axis of said imaging optical system to the laser-source-side end of an effective scanning range, W1 having a negative value; PA1 W2 represents a position (unit:mm) on said scanning target surface with respect to the optical axis of said imaging optical system to the other end of said effective scanning range, W2 having a positive value; and PA1 m is a natural number arbitrarily chosen based upon the number N of the reflection surfaces of the polygonal mirror.
In the scanning optical system disclosed in the JP publication described above, if the number of the reflection surfaces of the polygonal mirror is relatively small--eight (8) surfaces in the embodiment--the angle .alpha. can be set to a relatively large value, and therefore it allows relatively large freedom in designing the scanning optical system.
However, if the number of the reflection surfaces of the polygonal mirror is increased for high speed depicting, for example, to sixteen (16) or more, in order to satisfy the above equation, the value of the angle .alpha. becomes small, resulting in limiting the design freedom. For example, when the number N of the reflection surfaces is twenty-four (24), the distance D is 100 mm!, and the distance W is 20 mm!, the angle .alpha. becomes less than 18.5.degree., which is not a practical figure. It causes such a problem that the normal light beam to be incident upon the polygonal mirror interferes with the imaging optical system.